1. Field of the Invention
This invention is directed to the manufacture of polymeric hydrogel materials for use in ophthalmic devices and to the polymeric materials themselves both in the xerogel and hydrogel states.
2. State of the Art
Hydrogel materials are commonly used in the manufacture of what is commonly known as soft contact lenses. Hydrogel materials are a particular area of specialized chemistry and the material exhibits some unique properties. Generally, the hydrogel materials are copolymeric systems which are formed in the xerogel state where they are hard cross-linked materials. The xerogel in the presence of water hydrates and undergoes a change so that it attains the hydrogel state. Upon hydration, the resulting polymer composition contains water and, accordingly, becomes softer and more pliable as compared to the polymer composition prior to hydration.
It should be noted, however, that in the hydration of the xerogels the material (polymer) will undergo certain physical changes. Specifically, when the xerogel material is hydrated, the polymer undergoes considerable expansion/swelling (i.e., undergoes a volume increase). The exact amount of expansion is dependent upon several factors such as the nature and hydrophilicity of the polymer, the degree of cross-linking, and the like.
A majority of the known hydrogel materials used in contact lenses are cross-linked in structure including hydrogel materials based on hydroxyethylmethacrylate, a common component in hydrogel compositions used in contact lenses. Consequently, except for machining (e.g., lathing, cutting, etc.), these materials cannot be reshaped subsequent to polymerization. Moreover, these polymeric materials are cast in the xerogel state, and once hydrated become soft and pliable which makes machining these materials difficult. Accordingly, the final shape and other physical characteristics of the polymeric article are preferably formed during the xerogel state, i.e., prior to solvolysis and hydration.
As has been previously mentioned, the dimensions of the polymer changes upon hydration. Consequently, in the case of contact lenses and other ophthalmic devices where the proper dimensions of the hydrated product are essential to adequate performance of the product, the volume changes occurring during hydration have to be accounted for when forming/shaping the polymeric material in the xerogel state. As the volume changes increase upon hydration, so does the difficulty in accurately forming/shaping the polymeric material in the xerogel state so that upon hydration, the resulting hydrogel composition has the appropriate dimensions.
In addition to the above-mentioned problem regarding the volume change and the necessity of being able to accurately predict the dimensions of the product in the hydrogel state, there is also a problem in the fact that certain contact lens designs do not well tolerate significant volume change upon hydration. For example, the so-called SATURN.RTM. lens has a Rigid Gas Permeable (RGP) center with a concentric hydrogel skirt. This lens is fabricated with the concentric skirt in the xerogel form and hydrated after fabrication to the hydrogel form. However, the expansion/swelling of the skirt during hydration causes stresses at the boundary of the skirt and the RGP center as well as shape changes in the hydrogel skirt. This is because the RGP material is dimensionally stable and exhibits little, if any, expansion during hydration whereas the skirt material does. Consequently, there is differential expansion in the two materials and it is this differential expansion which causes stresses at the boundary of the skirt and the RGP center as well as the shape changes discussed above.
The need to compensate for swelling of polymeric materials during hydration is recognized in the art. For example, U.S. Pat. No. 4,093,361 describes the preparation of a hydrogel material which exhibits no net volume change upon hydration. In this case, polymerization of the monomer is conducted in the presence of a non-reactive water soluble neutral filler material. After polymerization is complete, the neutral filler material is washed out with a solvent thereby providing space for hydration of the polymer while leaving the final dimensions of the hydrogel unchanged from the xerogel state. Polymers made in this way, however, suffer the disadvantage that parameters such as hardness become unsatisfactory and the mechanical properties (e.g., modulus, tear strength, maximum elongation) of the hydrated resulting polymer are no longer optimum.
Additionally, European Patent Application Publication No. 0 495 603 A1 describes a process for providing a broad solution to the above-noted problems. Specifically, in this particular case, the hydrogel copolymer system is formed from two or more monomers of a first group and one or more monomers of a second group. The monomers of the first group each have one or more substitutable bulky leaving groups which can be removed by solvolysis to expose a hydratable moiety (e.g., a hydroxyl group) whereas the monomers of the second group do not.
After polymer formation, the leaving groups are readily removed in the presence of a mild base such as aqueous ammonium hydroxide and during subsequent hydration of the xerogel, the volume change resulting during hydration is controlled to from about 20 percent shrinkage to 40 percent expansion by controlling the make-up of the comonomer units and the size of the leaving groups. That is to say that the use of at least two different monomers having solvolyzable leaving groups permits some control over the volume change during hydration by the selection of the relative size of the leaving group, by the selection of the amounts of the two or more monomers relative to each other, and by the selection of the amount of monomer(s) of the second group relative to the total amount of the monomers of the first group.
In this regard, it is disclosed in this reference that the use of a combination of a 3:1 ratio of trichloroacetate ester of glyceryl methacrylate and trifluoroacetate ester of glyceryl methacrylate as the monomers of the first group in combination with methyl methacrylate as the monomer of the second group provides for essentially no volume change upon solvolysis and hydration.
Notwithstanding the advantages of this approach in controlling volume change during hydration, the use of at least two different monomers of the first group each containing solvolyzable leaving groups is not always advantageous from a processing point of view and requires that each of the monomers from the first group be compatible with each other and with the monomer of the second group.
In view of the above, it would be desirable to provide for polymer compositions which contain only one monomer having solvolyzable leaving groups in combination with one or more monomers which do not.
It would also be desirable to provide for polymer compositions which upon hydration and solvolysis yield a hydrogel with little or essentially zero volume change on removal of the leaving groups so that stresses and deformations arising during hydration of the material and especially of a composite article containing this material would be minimized.
Additionally, it would be desirable for the polymer compositions to have a high water content (i.e., from about 45 to about 56 percent by weight) so that the resulting polymer would be soft and pliable and therefore particularly suited for use in ophthalmic devices such as contact lenses.